A homeless man on a street corner -- a common sight in many cities

From Correspondent Martin Savidge

ATLANTA, (CNN) -- For many Americans, the nation's homeless also are faceless, and nameless.

Now a new federal study has set out to explain who the street people are and where they come from.

The Department of Housing and Urban Development study, the most comprehensive on homelessness to date, was financed by 12 federal agencies and features more than 4,200 interviews.

The scale of the problem is intimidating. In Atlanta, an estimated 14,000 to 21,000 homeless seek shelter each night.

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At the Shepherd's Inn, there is no more room. About 1,200 people are being fed but there are only beds for 400. A nearby church handles some of the overflow.

Cedric Graham, 35, is spending only his second night in the shelter. "I never thought that I would be in this situation at all, you know. I've always seen it from the outside looking in," he said.

Beds are full at this homeless shelter in Atlanta

Substance abuse brings many to the shelters, and it was 13 years of alcoholism that put Guy Pierri there. For five years, he's been trying to get off the street. But the lack of a permanent address -- and the phone that goes with it -- puts off potential employers.

"We'll get a message when we come in but sometimes we're too late to call them and they'll pass the job off to someone else," Pierri says.

The study does have some bright spots. Some programs to help the homeless find work are succeeding, the study found.